Continuing growth of the U.S. population is creating an increased demand for wastewater treatment capacity. Consequently, the amount of municipal sewage sludge (MSS) produced will increase in the foreseeable future. In 2004, the U.S. Environmental Protection Agency (U.S. EPA) estimated that approximately 6.5 million tonnes of MSS are produced each year in the U.S. (NEBRA 2007).
FIG. 1 shows the different categories and magnitudes of U.S. MSS uses. A first area 10 (farmlands), a second area 12 (horticulture and landscaping) and a third area 13 (land restoration silviculture) represent beneficial land applications. A fourth area 14 (landfill), a fifth area 16 (incineration) and a sixth area 17 (surface disposal only) represent disposal options. As shown, 55% of MSS is estimated to be applied on land for beneficial purposes, totaling approximately 3.6 million tonnes of MSS each year. Treated MSS fit for land application (also termed biosolids) is rich in organic carbon content and nutrients that can improve soil properties, crop productivity and fertility (Smith 1995, Wang et al. 2008). A Life Cycle Assessment (LCA) performed on different treatment and disposal scenario of MSS showed that the combination of anaerobic digestion and agricultural land application was the most viable option creating lower costs and consuming less energy compared to other treatment and disposal options (Suh and Rousseaux 2002). Landfilling of MSS is becoming difficult due to limited land availability, increased compliance costs, and concerns regarding leachate and greenhouse gas emissions from methane production (Wang et al. 2008). Incineration of MSS, on the other hand, involves high operational costs and concerns over toxic flue gas emissions (Lundin et al. 2004). Recent efforts in using MSS as a resource for energy production and nutrient recovery have increased the beneficial value of MSS (Rulkens 2007, Wang et al. 2008). Beneficial reuse of MSS already is applied to over half of the total U.S. MSS mass and this trend is expected to increase further in the foreseeable future.
However, a notable downside to the beneficial reuse of MSS is the presence of persistent (P), bioaccumulative (B), and toxic (T) chemicals, toxic metals and pathogens in MSS, all known to pose significant environmental and human health concerns (Chaney et al. 1996, Dowd et al. 2000, Gerba et al. 2002, National Research Council 2002).
The number of hazardous chemicals detected in MSS is considerable and constantly increasing (U.S. EPA 2009, Venkatesan et al. 2015, Venkatesan and Halden 2014a,b,c). Currently, the U.S. EPA has established standards for nine toxic compounds in land-applied MSS, all belonging to the inorganic group of metals, including As, Cd, Cu, Hg, Mo, Ni, Pb, Se, and Zn (40 CFR Part 503 of US Federal Regulations). The Part 503 regulation focuses on the presence of (a) nine metals, (b) microbial pathogens, and (c) MSS unwanted attraction to disease vectors as a basis for determining MSS quality.
Furthermore, in addition to the aforementioned concerns, the presence of significant quantities of organic contaminants of emerging concern (CECs) poses a newly recognized threat to the practice of land application of MSS (Chaney et al. 1996, Clarke and Smith 2011, Topp et al. 2008, Venkatesan et al. 2015). Many of these contaminants are known endocrine disruptors, carcinogens and potent toxicants to aquatic organisms. In addition, an emerging concern is the presence of significant quantities of important antibiotics in MSS (e.g. ciprofloxacin) that is known to promote antibiotic resistance in human pathogens even when present at nontherapeutic concentrations (Martins da Costa et al. 2006, Pruden 2013, and Reinthaler et al. 2003).
Prior screening by Arizona State University (ASU) for 239 contaminants in nationally representative samples of MSS collected from more than 160 U.S. wastewater treatment plants (WWTPs), showed the presence of 130 CECs in MSS (Venkatesan et al. 2015, Venkatesan et al. 2014a, 2014b). These included pharmaceuticals and personal care products (PPCPs), brominated flame retardants (BFRs), perfluorinated chemicals (PFCs), 4-nonylphenol and its ethoxylates (NP and NPEOs), as well as hormones, polybrominated dioxins and furans (PBDD/Fs), and nitrosamines. Chemicals detected were calculated to contribute about 0.04-0.15% of the total dry mass of MSS produced in the U.S. annually, a mass equivalent to 0.4-1.5 g/kg of dry sludge or about 4,700 tonnes (range: 2,600-7,900 tonnes) of chemicals annually. However, this estimate is conservative with respect to mass (i.e., lower than the true value), since other industrial organic chemicals known to occur in MSS (e.g., linear alkylbenzene sulfonates, PCBs, etc.) were not included in the screening. The lack of transformation of these contaminants during optimized biological wastewater treatment and sludge treatment processes (e.g., activated sludge treatment followed by anaerobic digestion), indicate a strong persistence and resistance to biotransformation of these compounds upon release into the environment (Venkatesan and Halden 2014a).
Another important concern is leaching of contaminants from land applied MSS. Recent studies have shown contamination of groundwater, surface water, and uptake of CECs by agricultural crops from soils amended with MSS (Clarke and Smith 2011, Lapworth et al. 2012, Sepulvado et al. 2011, Wu et al. 2010, Xia et al. 2010). NP and NPEOs, PFCs and PPCPs have been shown to leach from MSS at significant concentrations (Edwards et al. 2009, La Guardia et al. 2001, Lindstrom et al. 2011, and Sepulvado et al. 2011). PFC concentrations in well water and surface water resulting from contaminant leaching from nearby fields amended with MSS were shown to be in excess of U.S. EPA's health advisory level for drinking water (Lindstrom et al. 2011). This brings up two important questions addressed by the inventors in the solution presented herein: First, what fraction of the estimated contaminant load of 4,700 tonnes can readily leach from land-applied MSS to contaminate our water resources? And, secondly, is there a straightforward technology that can remove this leachable fraction of the contaminant mass from MSS prior to disposal on land? Current safety assessments of land-applied MSS are incomplete, as this information is not available. The methods and systems presented herein address this knowledge gap and introduce a straightforward process for removing the leachable fraction of contaminants from MSS to prevent adverse environmental and human health effects upon MSS recycling on land.
The present invention provides novel solutions for the deficiencies inherent in methods like those described above.